In counter-illumination, the lanternsharks, like many deep-sea animals, light up their undersides in order to disguise their silhouette when seen from below. Brighter bellies blend in with the light filtering down from the surface. (Related: "Glowing Pygmy Shark Lights Up to Fade Away.")

Fishing the 2-foot-long (60-centimeter-long) lanternsharks up from Norwegian fjords and placing them in darkened aquarium tanks, the researchers noticed that not only do the sharks' bellies glow, but they also had glowing regions on their backs.

The sharks have two rows of light-emitting cells, called photophores, on either side of a fearsome spine on the front edges of their two dorsal fins.

Study co-author Jérôme Mallefet explained how handling the sharks and encountering their aggressive behavior hinted at the role these radiant spines play.

"Sometimes they flip around and try to hit you with their spines," said Mallefet, also from Belgium's Catholic University of Louvain. "So we thought maybe they are showing their weapon in the dark depths."

To investigate this idea, the authors analyzed the structure of the lanternshark spines and found that they were more translucent than other shark spines.

This allowed the spines to transmit around 10 percent of the light from the glowing photophores, the study said.

For Predators' Eyes Only

Based on the eyesight of various deep-sea animals, the researchers estimated that the sharks' glowing spines were visible from several meters away to predators that include harbor seals (Phoca vitulina), harbor porpoises (Phocoena phocoena), and blackmouth catsharks (Galeus melastomus).

"The spine-associated bioluminescence has all the characteristics to play the right role as a warning sign," said Mallefet.

"It's a magnificent way to say 'hello, here I am, but beware I have spines,'" he added.

But these luminous warning signals wouldn't impede the sharks' pursuit of their favorite prey, Mueller's bristle-mouth fish (Maurolicus muelleri), the study suggested. These fish have poorer vision than the sharks' predators and may only spot the sharks' dorsal illuminations at much closer range.

For now, it remains a mystery how the sharks create and control the lights on their backs. The glowing dorsal fins could respond to the same hormones that control the belly lights, suggested Mallefet, but other factors may also be involved.

"MacGyver" of Bioluminescence

Several other species use bioluminescence as a warning signal, including marine snails (Hinea brasiliana), glowworms (Lampyris noctiluca), and millipedes (Motyxiaspp.).

Edith Widder, a marinebiologist from the Ocean Research and Conservation Association who was not involved in the current study, previously discovered a jellyfish whose bioluminescence rubs off on attackers that get too close.

"It's like paint packages in money bags at banks," she explained.

"Any animal that was foolish enough to go after it," she added "gets smeared all over with glowing particles that make it easy prey for its predators."